gosrc: Use filepath.Join to manipulate file paths.
gosrc: Use filepath.Join to manipulate file paths.

path.Join is meant to manipulate '/'-separated paths only. It was being
used incorrectly where filepath.Join should be used instead.

jeweljar/gddo,rjeczalik/gddo,rjeczalik/gddo,sourcegraph/gddo,rjeczalik/gddo,golang/gddo,segmentio/gddo,sourcegraph/gddo,jeweljar/gddo,golang/gddo,sourcegraph/gddo,jeweljar/gddo,rjeczalik/gddo,sourcegraph/gddo,golang/gddo,segmentio/gddo,segmentio/gddo,golang/gddo,jeweljar/gddo,segmentio/gddo